This invention relates generally to optical gain devices and, more specifically, to pump apparatus for optical gain devices using stimulated Raman scattering.
An optical gain medium is a device that, when provided with optical pumping energy, increases the amplitude of a desired optical signal. Optical gain media may be constructed using optical fiber, and used for fiber lasers or fiber-based optical amplifiers. One form of optical amplifier known in the art is based on stimulated Raman scattering (SRS), and is referred to generally as a xe2x80x9cRaman amplifier.xe2x80x9d In a Raman amplifier, optical pumping energy is injected into an optical fiber medium through which the signal to be amplified is propagated. The optical pumping energy, via SRS, allows for a transfer of optical power to a wavelength longer than the pumping wavelength, due to the excitation of a vibrational mode in the medium that provides gain at the longer wavelength. A Raman laser is similar, but has no signal to be amplified, and through SRS, simply develops the pump signal into an output signal of higher power and longer wavelength using a resonator structure.
The longer wavelengths to which optical power is transferred in a Raman gain medium may be predetermined relative to the wavelength of the pumping energy. These wavelength shifts are referred to as xe2x80x9cStokesxe2x80x9d shifts and, since each Stokes shift is a known amount relative to the wavelength from which the energy is transferred, a resulting xe2x80x9cRaman-shiftedxe2x80x9d wavelength may be selected by proper selection of the pumping wavelength. In communication systems, a number of different signals at different wavelengths are typically multiplexed over the same communications medium, e.g., an optical fiber. These different signal wavelengths, or xe2x80x9cchannels,xe2x80x9d are in a continuous band, and are separated by a minimum channel spacing necessary for good discrimination. When it becomes necessary to boost the signal power of the signals, all of the channels are typically amplified together in an optical amplifier. However, the useful gain bandwidth of such an amplifier is necessarily limited, and the number of channels that can be amplified simultaneously is therefore also limited.
In current systems, wavelength bands have been identified that have customarily been used for optical communications. The conventional band is referred to as the xe2x80x9cC-bandxe2x80x9d and is understood to span roughly a range from approximately 1530 nm to approximately 1565 nm. This has been the range to which optical amplifiers, typically erbium-doped fiber amplifiers (xe2x80x9cEDFAsxe2x80x9d), have been directed for operation. More recently, a band of longer wavelengths has come into favor for certain applications, and may be used either alone or in combination with the C-band. This band of longer wavelengths is referred to as the xe2x80x9cL-band,xe2x80x9d and is understood to span roughly a range from approximately 1565 nm to approximately 1610 nm. In the past, Raman amplifiers have been made that provide amplification in the L-band. Similarly, attempts have been made to build Raman amplifiers that provide wideband amplification across both the C-band and the L-band, with varying degrees of success. However, such amplifiers tend to be complicated and costly as compared to those which amplify in the C-band alone. Moreover, amplifiers that pump in the C-band and the L-band simultaneously can not also be used to efficiently pump just one band or the other.
In accordance with the present invention, an optical pumping apparatus provides pumping for just a first wavelength band, or for the first wavelength band and a second wavelength band together. In particular, an optical pumping apparatus provides optical pump energy to an optical gain medium that produces optical gain by stimulated Raman scattering. The gain is provided to each of a first signal wavelength range and a second signal wavelength range, the second range having longer signal wavelengths and not overlapping with the first. The pumping apparatus includes two pump units, a first pump unit that provides optical pump energy to the gain medium in a first pump wavelength range, and a second pump unit that provides pump energy to the gain medium in both the first pump wavelength range and a second pump wavelength range. Pumping in the first pump wavelength range results in signal gain in the first signal wavelength range, while pumping in the second pump wavelength range results in signal gain in the second signal wavelength range.
In addition to producing gain in the second signal wavelength range, pumping in the second pump wavelength range also tends to deplete the gain spectrum in the first signal wavelength range. This would ordinary cause a disruption in the gain bandwidth of the first signal wavelength range in that the overall flatness of the gain response would be compromised. However, the second pump unit of the present invention also provides pump energy that produces gain in the first signal bandwidth, and that compensates for any disturbances due to pumping for the longer wavelengths. Thus, the second pump unit can operate as an upgrade unit, in that it allows a user to extend the signal gain bandwidth beyond that of the first signal wavelength range, and still maintain the overall flatness of the gain response. That is, when the second pump unit is not activated, the pumping apparatus provides a gain response with good flatness over the first signal wavelength range. When the second pump unit is activated, the gain response is extended to the second signal wavelength range, and good flatness is maintained over both wavelength ranges.
The second pump unit may be in the form of a detachable module, to allow it to be connected and disconnected from the first pump unit. The pumping by the second pump unit so as to provide gain in both signal wavelength ranges is preferably such that with or without the use of the second pump unit, the flatness maintained across the gain spectrum is good enough that the signal gain does not deviate more than 1 dB from a pre-selected spectral shape within the designated wavelength band. That is, the flatness refers not just to maintaining an approximately constant signal gain across the designated wavelength band, but to preventing deviation from any chosen gain spectrum shape. The second pump unit also includes a first pump source and a second pump source. With regard to the degree to which the second pump source of the second pump unit pumps for the first signal wavelength, it is preferred that the second pump source does not provide direct signal gain in the second signal wavelength range any greater than one-half of the maximum signal gain that it produces.
In the preferred embodiment, the pump energy from the first pump unit and that from the second pump unit is combined by a pump combiner before being directed to the gain medium. This pump combiner may be, for example, a wavelength division multiplexer. Although the wavelength ranges in question may vary depending upon the application, the preferred embodiment of the invention is directed to providing gain in the conventional xe2x80x9cC-bandxe2x80x9d and xe2x80x9cL-band.xe2x80x9d For example, the first signal wavelength band may be from approximately 1530 nm to approximately 1565 nm, and the second signal wavelength band may be from approximately 1565 nm to approximately 1610 nm. Furthermore, although the wavelength outputs from the different pump sources should fit the conditions described above, it is recognized that a plurality of pump sources may be used together to provide any of the necessary wavelength outputs, such as the optical energy output from the first pump unit.